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Derivatives of 1,2-diketones; Emission spectra in polymer matrices and efficiency as initiators of degradation
Polymer Degradation and Stability 43 (1994) 195-201
Derivatives of 1,2-diketones; Emission spectra in polymer matrices and efficiency as initiators of degradation Pavol Hrdlovi~ & Ivan Luk~i~ Polymer Institute, Slooak Academy of Sciences, Sk-842 36 Bratislava, Dtibravskd cesta 9, Slooak Republic (Received 9 June 1993; accepted 19 July 1993)
The absorption and emission spectra of derivatives of benzil and 1phenylpropane-l,2-diones in solution and polymer matrices like polyethylene, polypropylene, polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) have been investigated. The longest wavelength absorption band, of low intensity, of all acyclic derivatives shifts hypsochromicaily in going from nonpolar to polar solvents, which is typical for n - n * transitions. The most intense second band, of n - n * type, shifts bathochromically. Emission from 1,2-diketonones is influenced slightly by the polymer matrix for polyethylene, polypropylene and polystyrene at 77 K. In polar matrices a slight hypsochromic shift is observed, with broadening of the emission band. The lifetime of emission (phosphorescence) lies in the range 2-5 ms, and the decay proceeds monoexponentially. At laboratory temperature the decay is more rapid and might be fitted by a biexponential. The mean lifetime is about 0.3 ms in matrices with Tg above laboratory temperature. In polyolefins, no emission is observed at laboratory temperature. The most effective initiator of photooxidation is the derivative with two chromophoric groups.
(PVC) has been evaluated and compared with that of 2-methylanthraquinone and benzophenone. 9 The goal of this study is to understand, at least qualitatively, the influence of the polymer environment on photophysical and photochemical processes of derivatives of 1,2-diketones.
INTRODUCTION Derivatives of 1,2-diketones related to biacetyi and benzil are widely used as photoinitiators of various radical processes in which their photoreactivity is employed. 1.2 They are used as donors or acceptors or triplet energy. This process can be monitored by quenching or sensitization of emission. 3'4 The photochemical reactivity of low molecular weight 1,2-diketones has been thoroughly studied. 5 These chromophores have also been bound to macromolecules. 6-9 Macromolecules marked by 1,2-diketones have been used for the study of different processes. In order to use these probes effectively their behaviour in different environments should be known. Therefore, absorption and emission spectra of acyclic 1,2-diketones in solution and polymer matrices have been compared with the cyclic one. Their efficiency as initiators of photooxidation of polyolefins, polystyrene and poly(vinyl chloride)
EXPERIMENTAL Chemicals The structures of derivatives of 1,2-diketones and related compounds are given in Table 1. 1,2-Diphenylethane dione (I, benzil) was zone refined (Lachema, Brno, Czech Republic). The following derivatives of 1,2-diketones, prepared as previously described, 7'"' were used. 1- [4- (2-Hydroxyethoxy)phenyl]- 2- phenylethanedione (II), m.p. 101-103°C; 1-[4-(2-acetyloxy195
196
Pavol Hrdlovi6, Ivan Lukd6
Table 1. The structure of derivatives of 1,2-diketones: RI--CO---CO---~R2 Substrate ! !! !!! IV V V! VII Vill IX X
1,2-Diphenylethanedione (bcnzil) 1-Phenyl-2-[4-(2-hydroxycthoxyphenyl)]ethandione 1-Phenyl-2-[4-(2-acetyloxyethoxyphenyl)]ethanedione 1-Phenyl-2-14-(3-chloropropanoylphenyl)]et hanedionc l-[4-(2-Hydroxycthoxy)phenyl]propanc- 1,2-dione 1-[4-(2-Acctyloxyethoxy)phenyl]propane- 1,2-dionc 1,2-Difurylethancdione (1,1'-furil) 9,10-Phenanthrencquinone 2-Methyl-9,10-anthraquinone Bcnzophcnonc
RI
R2
C~,H5
H
Coil5
OCH2CH20H
C6H5
OCH2CH2OCOCH 3
Coil5
COCH2CH2CI
CH 3
OCH2CH2OH
CH 3
OCH2CH2OCOCH 3
prepared by casting were of good optical quality. At higher concentrations of dopant, some inhomogeneities were observed due to microcrystals. Those films were not used. The film thickness was about 50/zm. Polyethylene (PE) and isotactic polypropylene (iPP) were prepared by hot pressing of powder impregnated with low molecular weight dopants. The impregnation was accomplished by mixing polymer powder (2g), dopant (10mg) and dichloromethane (10ml) and allowing the mixture to stand for 24h. The solvent was then removed, and the mixture was homogenized and used for hot pressing. PE films were pressed at 5MPa and 140°C and iPP films at 190°C for 2 min. The thickness was 0-1 to 0.15 mm.
Spectral measurements ethoxy)phenyl]-2-phenylethanedione (III), m.p. 70-71°C; 1-[4-(3-chloropropanoyl)phenyl]-2phenylethanedione (IV), m.p. 71-73°C; "~ 1[4 - (2 - hydroxyethoxy)phenyl]propane - 1,2 - dione (V), m.p. 36-38°C; and 1-[4-(2-acetyloxyethoxy)phenyl]propane-l,2-dione (VI), m.p. 2930°C. 1,1'-Difurylethanedione (VII, 1,1'-furil), m.p. 164-164.5°C was prepared according to Ref. 11 and crystallized from methanol. 9,10Phenanthrenequinone (VIII), 2-methyl-9,10anthraquinone (IX) and benzophenone (X) were products of Fluka, Buchs, Switzerland, and Suchard, Munich, Germany, respectively. Cyclohexane was of UV spectroscopy grade (Merck, Darmstadt, Germany) Ethanol was of UV spectroscopy grade and chloroform was of analytical-reagent grade (Lachema). The tetrahydrofuran used was pure (Ubichem, UK).
Preparation of films Polymer films were prepared by casting from solutions or by pressing of polymer powder at higher temperature. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) films were prepared by casting 1 ml of a solution containing about 1 mg compound in chloroform solution of polymer (5 g per 100 ml) on a glass plate (2-5 cm x 3 cm) and allowing the solvent to evaporate. For casting of PVC films, tetrahydrofuran was used as solvent. For casting poly(vinyl acetate) (PVA) films, a mixture of methanol and water was used. Only compounds II and V yielded optically clear PVA films. Films
Absorption spectra were recorded on a Specord M-40 (Carl Zeiss, Jena, Germany) in films and solutions in the concentration range 10 -1 to 10 -5 mol dm -3. Emission spectra were measured on a system constructed from available optical and electronic parts as previously described in Ref. 4. The MEK 100 was, however, substituted by a DC amplifier of our own construction, based on operational amplifiers, and the DC output was fed to an analogue-to-digital converter, which was interfaced with a microcomputer. 12 An end-window photomultiplier (EMI6256S) was used as a detector. The emission spectra were compared in the same region. Emission spectra of polymer films at 77K and laboratory temperature were recorded in the same way as previously. 13 The decay of emission was measured in the same set-up after single pulse excitation, as in Ref. 13. The data were fitted to mono- or bi-exponential decay, based on the algorithm described by Hrn(:irik,14 without deconvolution of the excitation pulse.
Photooxidation of polymers Films of polymers, prepared in the same way as for spectral measurements, were exposed to filtered radiation of a medium pressure mercury arc RVL 125 W (Tesla, Praha, Czech Republic) as described in Ref. 9. The course of photooxidation was monitored by IR spectroscopy (IR 75; Carl Zeiss) in the region of valence vibrations of carbonyl groups.
Derivatives of 1, 2-diketones as initiators of polymer degradation
197
RESULTS A N D DISCUSSION Absorption spectra
4.0
Absorption spectra of acyclic 1,2-diketones, derived from propane-l,2-dione and benzil, usually exhibit three absorption bands in the regions around 400,290 and 225 nm (Table 2). A band around 260nm appears for some derivatives. The longest wavelength band, around 400 nm, has low intensity (e -" 100dm 3 mol -~ cm -~) and is well resolved only in nonpolar solvents. In chloroform, this band shifts slightly hypsochromicaUy. In polar solvents, this shift is large. As a result, this low intensity band is overlapped by the more intense band at 290nm. This characteristic is typical for n-~r* transition of monoketones like benzophenone. The long wavelength band of I is preserved, even in polar solvents. The changes of this band for 1 as a result of solvent polarity were interpreted as due to formation of a weak
Table 2. Absorption spectra of 1,2-diketones No." 1
II
I!!
IV
Solvcn('
Am~,~'(rim) (log e)
ET CH CY
21214-141
226 (4.00)
261 (4.43) 265 (4-45) 259 (4.57)
380 (1.89) 382 (1.86) 392 (1.74)
ET CH CY d
225 14-191
256 (4.30)
295 (4.44) 296 (4.43) 285
385s (2.04) 385s (2.05) 392
225
253
ET CH CY a
222(3-91)
258(3.97) 254
295 (4.11) 293 (4-47) 286
385s (1.78) 385s (2-04) 392
225
268 (4.52) 270 (4-48) 271 (4-48)
334s (2.77) 334s(2.56)
401s (1.96) 392 (2-03) 415 (4.48)
295 (4.21) 300 (4.20) 294
408 (1.74) 400s (1.79) 412
299 (4.02) 299 (4.(15) 296 (4.1121
41~)s
223 (4-26)
ET CH CY
V
ET CH CY 't
22414-01)
V!
ET CH CY
232 (3-67)
VII
ET CH CY
225
224(3.89)
409 (1.791
304 (4.03) 306 14-18) 302 (4.26)
405 (2.08) 427 (1.94)
VIII
ET CH CY
257 (4.32)
266 (4-34) 268(4-64)
324 (3.50) 325 (3.67) 321
418 (3.03) 420 (3-23) 400
IX
ET CH CY
257(4-64) 259 (4.56) 256(4.8111 40(1 (2.16)
274(4.11) 27914.(181 275 (4.23) 420 (2,00)
328 (3.63) 334 (3.55) 322 (3-72)
400 (2-23) 400 12.121 377 (2.32)
"Compounds are designcd according to Tablc 1. b ET: cthanol; CH: chloroform: CY: cyclohcxanc. ~ s: shoulder. ,t Partially dissolved; the saturated solution was measured.
i
~3.0 o
I~ 2.0
1.0
I
40
I
30 v x 103(cm -1)
I
20
Fig. 1. Absorption spectra of IV in (1) cyclohexane and (2) ethanol. 1-polar solvent complex. 15 VII exhibits a very distinct n - : r * band in cyclohexane. In ethanol it is completely lost. Similarly to Vll, the compound 1V loses its low intensity band in polar solvents (Fig. 1), but a small shoulder is preserved, probably due to the second chromophoric carbonyl group. The compounds IV and VII should form a weak complex with polar solvents even more easily than 1.15 Other absorption bands are more intense and less sensitive to solvent polarity. A slight bathochromic shift is observed in going from nonpolar to polar solvents. This behaviour is also observed for the 260 nm band. In contrast, Vlll, which is a cyclic 1,2diketone, exhibits a longest wavelength band of higher intensity (e-- 103 dm 3 mo1-1 cm -~) than acyclic 1,2-diketones. In going from nonpolar to polar solvent the bathochromic shift of this band is observed in solution as well as in the polymer matrix. In this respect its intensity is higher than for the typical n - : r * transition of acyclic 1,2-diketones. 1,2-Diketones exhibit similar absorption spectra in polymer matrices and solvents of similar polarity. No aggregation was observed in the absorption spectra in our concentration range of doping of 1,2-diketones in polymer matrices. Since the differences in polarity of polymer matrices are low, the differences in absorption spectra are low as well. Therefore, the differences in photoreactivity of 1,2-diketones in
198
Pavol Hrdlovi~, Ivan Lukd~
various matrices are not expected to be large either. Emission spectra Simple acyclic 1,2-diketones exhibit measurable emission in deaerated solution at laboratory temperature) At low temperature (77 K) in the solid solution, they yield very intense emission. For optical measurements of this type, PMMA constitutes a suitable environment. PMMA films of 50 # m thickness are mechanically stable and they withstand sudden temperature changes. On the other hand, control of impurities and oxygen seems to be a problem. Therefore, it is difficult to compare absolute intensities of compounds doped in a polymer film. It is appropriate to compare the intensity of emission of a series of compounds with the intensity of emission of a selected compound which is not sensitive to environment and quenching. Anthracene might be a suitable standard. At front face arrangement and high absorption at the excitation wavelength, the relative quantum yield, given as the ratio of integrals of emission bands of the compound under study and a standard, does not depend on the condition of measurement. Thus, the comparison of relative intensities of emission can yield important information. Emission spectra of 1,2-diketones of the benzil type at 77 K and at laboratory temperature are simple and exhibit a broad band (Fig. 2). Derivatives of 1-phenylpropane-l,2-dione exhibit another rather weak band around 570 nm (Table 3). In this region there is a maximum of the single emission band of cyclic VIII (Fig. 2). The excited triplet state of VIII has the same structure as the ground state, namely c/s planar, because it is fixed by the frame of the phenanthrene. On the other hand, the structure of the lowest triplet state of acyclic 1,2dicarbonyl derivatives is different, namely trans planar. ~6 Recently, Mukai et al. ~7 have classified three different configurations for the first triplet state and found a correlation between the parameters of zero field splitting and the maximum of phosphorescence. According to their interpretation the longest wavelength phosphorescence at 570 nm originates in a trans planar structure. This means that the emission from T~ of trans and c/s planar structure is similar at 570 nm due to extensive conjugation. Emission at lower values of ~,m,x stems from structures
II
tl
II I\i i / / I! i
500 ;~ (nrn)
400
600
Fig. 2. Emission spectra of (1) !I!, (2) VI and (3) VIII in PMMA at 77 K.
Table 3. E m i s s i o n spectra of 1,2-diketones in PMMA film at 77 K and laboratory temperature Compound ~
Temp. (K)
;t.,ax (nm)
dprc
MLT/r
a
(ms)
I
Ith 77
544 545,576
0.010 0-116
0-12 5-5
11
It 77
519
0.035 1.393
0.27 3.8
111
It 77
533 522
0.037 1-984
0-18 4.1
IV
It 77
544 529
0-032 2.201
0.42 5.6
V
it 77
527,527 524, 546
0-064 0.659
0-48 2.3
V!
It 77
529 528,561
0-063 0.567
0.49 2-3
VII
It 77
517, 566 519,566
0.039 0.879
0.19 5.4
ViII
It 77
394,403,546,574 569
0-038 0"802
9.0
iX
It 77
428, 438,469, 492,529 445,490, 526
0-078 2-483
X
it 77
418, 438,449,453,470 414,448,476, 510
0-01 2.017
= Designation of compounds according to Table 1. b It: laboratory temperature (293-298 K). c Quantum yield relative to the emission of anthracene in PMMA film (50 #m) at 0.1 mol kg -I concentration at 313 nm excitation. a MLT: mean lifetime of emission at laboratory temperature which was fitted to a biexponential; the decay of emission at 77 K was fitted to a monoexponential.
199
Derivatives of 1, 2-diketones as initiators of polymer degradation Table 4. Emission spectra of 1,2-Diketones in PE a Compound
T e m p . (K)
!
It 77
531
0.012 0.855
5.2
It 77
545 524
0-016 5.15
3.4
It 77
512
0-027 3.695
4.2
it 77
550
0.009 2.010
4-3
It 77
528,566
0.027 1.480
1.4
It 77
530,575
0-008 0.690
1.8
it 77
522,572
0.003 0.565
3.5
I! !!I IV V V! 400
500 ~. (nm)
600
VII
;tm~x(nm)
Cr h
zc (ms)
Fig. 3. Emission spectra of V I I in (1) P M M A at laboratory temperature, (2) P M M A at 77 K and (3) P E at 77 K.
See footnotes to Table 3. b Q u a n t u m efficiency relative to emission of anthracene. c Decay of emission at 77 K was fitted to a monoexponential.
where the two benzoyl groups are mutually rotated. Consequently, we cannot identify any T~ state of the 1,2-diketones under study, in any matrix, as trans planar, because all Am~x were lower than 570 nm. The compound IV exhibited the highest ~max. This implies that all structures of T~ have got some value of angle between the two benzoyl groups, which lies between 0° and 90 °. This angle changes more or less continuously because it was not possible to identify clearly any distinct states. It is of some interest to note that compounds II! and VII exhibit low ~'max in PE and PMMA matrices, indicating a rather large angle between benzoyl groups, which might be due to some specific interactions. In going from laboratory temperature to 77 K, a small hypsochromic shift and a narrowing of the emission bands is observed (Fig. 3). This again implies that the emission at 77 K originates from structures with a larger angle between the planes of the benzoyl groups. The quantum yields of 1,2-diketone emission relative to anthracene are rather low at laboratory temperature (Tables 3 and 4). On the other hand, the emission at 77 K is comparable with that of anthracene and in some cases even higher, especially in PE matrix. At the moment it is difficult to decide whether the increase in emission intensity is due to suppressing some specific radiationless process or lack of quenching by impurities or oxygen at 77 K.
Values of ~, for 1,2-diketones in the PE matrix at laboratory temperature are very low (Table 4). This indicates that oxygen quenching is the dominating process, leading to the decrease in intensity. This conclusion supports the fact that the intensities from 1,2-diketones in PMMA and PE are similar at 77 K. At laboratory temperature the difference in intensities of 1,2-diketones is caused by different extents of oxygen quenching in polycrystaUine PE and glassy PMMA. In Table 5, the maxima of emission in various matrices are given. The general trend in these values is clearly seen for derivatives of IV and V. In going from nonpolar to polar matrices a Table 5. The influence of polymer matrix on maximum of emission of 1,2-diketones at 77 K ° Compound
I n nl iv v v! vii viii
Matrix h PE
iPP
PS
531 524 512 550 528 530 522
543 527 533 540 528 528 522
539 526 535 538 530 528 522 577
"See footnotes to Table 3. b See experimental section.
PMMA (~,. . . . nm) 545 519 522 529 524 526 519 567
PVC
545 518 518 534 525 525 518 572
PVA
519
523
200
Pavol Hrdlovi6, loan Lukd~
hypsochromic shift of the emission maxima is observed. For derivative IV this shift is about 20 nm. For other derivatives it is 5 to 10 nm only. There are, however, some deviations from this general trend, especially for III and I. The values of '~'max for these compounds are rather low as one would expect for a PE matrix. This means that these compounds adopt a structure where the angle between benzoyl groups is for some reason larger. In the polar matrix PVA, only the emission of II and V can be measured because the other derivatives are not soluble in water-ethanol mixtures. In this polyhydroxylic matrix the emission was similar to the ones in other polymer matrices except for a small hypsochromic shift. No emission below 450 nm for these compounds was observed as reported by Inoue et al. 18 for so-called 'bridged benzil' (10,11-dihydro-5H-dibenzo-[la, d]-cycloheptane10,11-dione) in a solid ethylene glycol matrix. We assume that the hypsochromic shift of emission in going from nonpolar to polar matrix is a polarity effect. The anomalies in the position of the emission maximum are due to matrix rigidity or some other specitic interactions. The decay of the emission at 77 K in PE and PMMA is monoexponential, covering two orders of magnitude in intensity. The correlation coefficient is usually better than 0-99 and the 'goodness of fit', expressed as the standard deviation of experimental and calculated curves is better than 5%. In the PE matrix the lifetimes are slightly shorter than in PMMA, which might indicate some contribution of static quenching. The lifetimes of 1-phenylpropane-l,2-diones are shorter, in the range 2-4 ms, as compared with I derivatives, which are in the range 3-6 ms. Since the measurable emission is observed in the P M M A matrix at laboratory temperature only, the decay of emission was analysed in this case only. The decay of emission was not monoexponential. It was well fitted by a biexponential decay. The 'goodness of fit', expressed as the standard deviation of experimental and calculated curves, is better than 2%. The mean lifetime lies in the range 0.1-0-5 ms, which is one-tenth of the value at 77 K. In this case, derivatives of 1phenylpropane-l,2-dione exhibit longer mean lifetimes than benzil-type derivatives. The decay was also fitted to the so-called 'stretched exponential'. The fit was, however, worse than for the biexponential. It is not quite clear which
temperature dependent radiationless process causes the decay to become different at different sites in the polymer matrix. It is possible that the accesibility of different sites, even in amorphous PMMA, to quencher (oxygen) varies. This has been recently observed for the quenching of phenanthrene in P M M A matrix by oxygen. 19 There is some distribution of the rate of quenching, but the biexponential could be a good approximation. Consequently, the decay of emission in polymer matrices at laboratory temperature is a complex phenomenon.
Photooxidation of polymers Derivatives of 1,2-diketones of both types are initiators of polymer photooxidation. They act as promotors of oxidation, mainly at the beginning (Fig. 4). In later stages of oxidation the process becomes slower. This course of oxidation is similar to that of other types of photoinitiator, e.g. derivatives of 9,10-anthraquinone.~° In order to compare various photoinitiators in different polymer substrates, it is convenient to define the effectiveness of acceleration. This is the ratio of the times required to reach a given absorption in a selected spectral region in polymer without and with initiator. A value larger than 1 means acceleration. If oxidation slows down at later stages, then the relative acceleration is influenced by the chosen limit. If AAco is chosen in the range 0.05-0.2, then the conclusions about the effectiveness of a series of photoinitiators might be quite reasonable. Comparison of relative values of acceleration in Table 6 shows that the derivatives of 6 0.50.4-
3 •
o
2 • 0.3 0.2 0.I
01
I00
i 200 t(h)
I 300
Fig. 4. The course of photooxidation of PE at laboratory temperature (1) without photoinitiators, and with (2) II!, (3) Vi, (4) IV, (5) VIII and (6) IX.
Derivatives of 1, 2-diketones as initiators of polymer degradation Table 6. Relative efficiency of 1,2-diketones and other initiators on photooxidation of polymers"
Compound
Matrixh PE
Without i ii iil
IV V ~i iX
X
iPP PS (relative efficiency)
201
less effective in P V C only. Similar effectiveness is exhibited by VII. On the other h a n d , derivatives with free hydroxy groups exhibit low effectiveness.
PVC
1.0 1-6
1.0 1-2
1.0 2.1
1.0 0.9
2-6
0-8
1.3
2.5
1-5 4.4 1.6 1.6 4-4 6.8 3.2
0-8 1.7 0.8 0.8 1.4 1.9 1.9
1.3 3.5 4-2 1.4 3.2 16-4 5-0
1.7 1.8 2.5 0.6 3.6 3.4 3.6
"See footnotes to Table 3. h PE exhibited absorption of carbonyl band AA~o= 0-2 after 300h, iPP after 70h, PVC after 375 h and PS exhibited AA~, =0.1 after 575 h.
1,2-diketones are less effective than 2-methylanthraquinone. For inherently more stable polymers like PS, the relative effectiveness is higher than in the more sensitive iPP. In this substrate, the effectiveness is even lower than 1 for four substrates. This means that, instead of acceleration, weak stabilization takes place. On the other h a n d , the stabilization effect is low, although the absorption in the n e a r - U V region is rather high. This is due to the fact that 1,2-diketone decomposes at the beginning rapidly. This is evident from I R spectra. The further course of degradation depends on the products formed, which are given by the type of 1,2-diketone and type of polymer. The most effective acyclic 1,2-diketone is the bichromophoric c o m p o u n d IV, which is slightly
REFERENCES
1. McGinniss, V. D., In Developments in Polymer Photochemistry, ed. N. S. Allen. Vol. 3, 1982, p. 1. 2. Fouassier, J. P., In Photochemistry and Photophysics, ed. J. F. Rabek. CRC Press, Boca Raton, FL, 1990, Vol. 2, p. 1. 3. Backstrom, H. J. L. & Sandros, K., Acta Chem. Scand., 14 (1960) 46. 4. Dan6i~ek, J., Hrdlovi~, P. & Luke,, I., Eur. Polym. J., 12 (1976) 513. 5. Rubin, M. B., Top. Curr. Chem., 129 (1985) 1. 6. Horie, K. & Mita, I., Macromolecules, 11 (1978) 1175. 7. Luk~i~,I., Zvara, I. & Hrdlovi~, P., Eur. Polym. J., 18 (1982) 427. 8. Gebert, M. S. & Torkelson, J. M., Polymer, 31 (1990) 2402. 9. Hrdlovi(~,P., Taimr, L. & Pospi~il, J., Polym. Deg. and Stab., 25 (1989) 73. 10. Luke,., I. & DasMohapatra, G. K., Collect. Czech. Chem. Commun., 57 (1992) 1082. 11. Vogel, A. I., Practical Oraganic Chemistry, Longman, London, 1956, p. 835. 12. Moyze, G., Ml~nek, J., Jur6~ik, D. & Hrdlovi~, P., Chem. Listy, 86 (1992) 57. 13. Hrdlovi~, P., Polym. Photochem., 7 (1986) 359. 14. Hrn~irik, F., Chem. PrCtm., 31/56 (1981) 594. 15. Miata, K. & Koyanagi, M., Bull. Chem. Soc. Jap., 61 (1988) 3813. 16. Morantz, D. J. & Wright, A. C. J., J. Chem. Phys., 54 (1971) 523. 17. Mukai, M., Yamauchi, S. & Hirota, N., J. Phys. Chem., 96 (1992) 9328. 18. lnoue, H., Sakurai, T., Hoshi, T., Okubo, J. & Kawashima, T., J. Chem. Soc., Faraday Trans., 2 (1986) 523. 19. Bagryansky, V. A., Korelev, V. V., Tolkatchev, V. A. & Bazhin, N. M., J. Polym. Sci., B: Polym. Phys., 30 (1992) 951.
Derivatives of 1,2-diketones; Emission spectra in polymer matrices and efficiency as initiators of degradation Pavol Hrdlovi~ & Ivan Luk~i~ Polymer Institute, Slooak Academy of Sciences, Sk-842 36 Bratislava, Dtibravskd cesta 9, Slooak Republic (Received 9 June 1993; accepted 19 July 1993)
The absorption and emission spectra of derivatives of benzil and 1phenylpropane-l,2-diones in solution and polymer matrices like polyethylene, polypropylene, polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) have been investigated. The longest wavelength absorption band, of low intensity, of all acyclic derivatives shifts hypsochromicaily in going from nonpolar to polar solvents, which is typical for n - n * transitions. The most intense second band, of n - n * type, shifts bathochromically. Emission from 1,2-diketonones is influenced slightly by the polymer matrix for polyethylene, polypropylene and polystyrene at 77 K. In polar matrices a slight hypsochromic shift is observed, with broadening of the emission band. The lifetime of emission (phosphorescence) lies in the range 2-5 ms, and the decay proceeds monoexponentially. At laboratory temperature the decay is more rapid and might be fitted by a biexponential. The mean lifetime is about 0.3 ms in matrices with Tg above laboratory temperature. In polyolefins, no emission is observed at laboratory temperature. The most effective initiator of photooxidation is the derivative with two chromophoric groups.
(PVC) has been evaluated and compared with that of 2-methylanthraquinone and benzophenone. 9 The goal of this study is to understand, at least qualitatively, the influence of the polymer environment on photophysical and photochemical processes of derivatives of 1,2-diketones.
INTRODUCTION Derivatives of 1,2-diketones related to biacetyi and benzil are widely used as photoinitiators of various radical processes in which their photoreactivity is employed. 1.2 They are used as donors or acceptors or triplet energy. This process can be monitored by quenching or sensitization of emission. 3'4 The photochemical reactivity of low molecular weight 1,2-diketones has been thoroughly studied. 5 These chromophores have also been bound to macromolecules. 6-9 Macromolecules marked by 1,2-diketones have been used for the study of different processes. In order to use these probes effectively their behaviour in different environments should be known. Therefore, absorption and emission spectra of acyclic 1,2-diketones in solution and polymer matrices have been compared with the cyclic one. Their efficiency as initiators of photooxidation of polyolefins, polystyrene and poly(vinyl chloride)
EXPERIMENTAL Chemicals The structures of derivatives of 1,2-diketones and related compounds are given in Table 1. 1,2-Diphenylethane dione (I, benzil) was zone refined (Lachema, Brno, Czech Republic). The following derivatives of 1,2-diketones, prepared as previously described, 7'"' were used. 1- [4- (2-Hydroxyethoxy)phenyl]- 2- phenylethanedione (II), m.p. 101-103°C; 1-[4-(2-acetyloxy195
196
Pavol Hrdlovi6, Ivan Lukd6
Table 1. The structure of derivatives of 1,2-diketones: RI--CO---CO---~R2 Substrate ! !! !!! IV V V! VII Vill IX X
1,2-Diphenylethanedione (bcnzil) 1-Phenyl-2-[4-(2-hydroxycthoxyphenyl)]ethandione 1-Phenyl-2-[4-(2-acetyloxyethoxyphenyl)]ethanedione 1-Phenyl-2-14-(3-chloropropanoylphenyl)]et hanedionc l-[4-(2-Hydroxycthoxy)phenyl]propanc- 1,2-dione 1-[4-(2-Acctyloxyethoxy)phenyl]propane- 1,2-dionc 1,2-Difurylethancdione (1,1'-furil) 9,10-Phenanthrencquinone 2-Methyl-9,10-anthraquinone Bcnzophcnonc
RI
R2
C~,H5
H
Coil5
OCH2CH20H
C6H5
OCH2CH2OCOCH 3
Coil5
COCH2CH2CI
CH 3
OCH2CH2OH
CH 3
OCH2CH2OCOCH 3
prepared by casting were of good optical quality. At higher concentrations of dopant, some inhomogeneities were observed due to microcrystals. Those films were not used. The film thickness was about 50/zm. Polyethylene (PE) and isotactic polypropylene (iPP) were prepared by hot pressing of powder impregnated with low molecular weight dopants. The impregnation was accomplished by mixing polymer powder (2g), dopant (10mg) and dichloromethane (10ml) and allowing the mixture to stand for 24h. The solvent was then removed, and the mixture was homogenized and used for hot pressing. PE films were pressed at 5MPa and 140°C and iPP films at 190°C for 2 min. The thickness was 0-1 to 0.15 mm.
Spectral measurements ethoxy)phenyl]-2-phenylethanedione (III), m.p. 70-71°C; 1-[4-(3-chloropropanoyl)phenyl]-2phenylethanedione (IV), m.p. 71-73°C; "~ 1[4 - (2 - hydroxyethoxy)phenyl]propane - 1,2 - dione (V), m.p. 36-38°C; and 1-[4-(2-acetyloxyethoxy)phenyl]propane-l,2-dione (VI), m.p. 2930°C. 1,1'-Difurylethanedione (VII, 1,1'-furil), m.p. 164-164.5°C was prepared according to Ref. 11 and crystallized from methanol. 9,10Phenanthrenequinone (VIII), 2-methyl-9,10anthraquinone (IX) and benzophenone (X) were products of Fluka, Buchs, Switzerland, and Suchard, Munich, Germany, respectively. Cyclohexane was of UV spectroscopy grade (Merck, Darmstadt, Germany) Ethanol was of UV spectroscopy grade and chloroform was of analytical-reagent grade (Lachema). The tetrahydrofuran used was pure (Ubichem, UK).
Preparation of films Polymer films were prepared by casting from solutions or by pressing of polymer powder at higher temperature. Poly(methyl methacrylate) (PMMA) and polystyrene (PS) films were prepared by casting 1 ml of a solution containing about 1 mg compound in chloroform solution of polymer (5 g per 100 ml) on a glass plate (2-5 cm x 3 cm) and allowing the solvent to evaporate. For casting of PVC films, tetrahydrofuran was used as solvent. For casting poly(vinyl acetate) (PVA) films, a mixture of methanol and water was used. Only compounds II and V yielded optically clear PVA films. Films
Absorption spectra were recorded on a Specord M-40 (Carl Zeiss, Jena, Germany) in films and solutions in the concentration range 10 -1 to 10 -5 mol dm -3. Emission spectra were measured on a system constructed from available optical and electronic parts as previously described in Ref. 4. The MEK 100 was, however, substituted by a DC amplifier of our own construction, based on operational amplifiers, and the DC output was fed to an analogue-to-digital converter, which was interfaced with a microcomputer. 12 An end-window photomultiplier (EMI6256S) was used as a detector. The emission spectra were compared in the same region. Emission spectra of polymer films at 77K and laboratory temperature were recorded in the same way as previously. 13 The decay of emission was measured in the same set-up after single pulse excitation, as in Ref. 13. The data were fitted to mono- or bi-exponential decay, based on the algorithm described by Hrn(:irik,14 without deconvolution of the excitation pulse.
Photooxidation of polymers Films of polymers, prepared in the same way as for spectral measurements, were exposed to filtered radiation of a medium pressure mercury arc RVL 125 W (Tesla, Praha, Czech Republic) as described in Ref. 9. The course of photooxidation was monitored by IR spectroscopy (IR 75; Carl Zeiss) in the region of valence vibrations of carbonyl groups.
Derivatives of 1, 2-diketones as initiators of polymer degradation
197
RESULTS A N D DISCUSSION Absorption spectra
4.0
Absorption spectra of acyclic 1,2-diketones, derived from propane-l,2-dione and benzil, usually exhibit three absorption bands in the regions around 400,290 and 225 nm (Table 2). A band around 260nm appears for some derivatives. The longest wavelength band, around 400 nm, has low intensity (e -" 100dm 3 mol -~ cm -~) and is well resolved only in nonpolar solvents. In chloroform, this band shifts slightly hypsochromicaUy. In polar solvents, this shift is large. As a result, this low intensity band is overlapped by the more intense band at 290nm. This characteristic is typical for n-~r* transition of monoketones like benzophenone. The long wavelength band of I is preserved, even in polar solvents. The changes of this band for 1 as a result of solvent polarity were interpreted as due to formation of a weak
Table 2. Absorption spectra of 1,2-diketones No." 1
II
I!!
IV
Solvcn('
Am~,~'(rim) (log e)
ET CH CY
21214-141
226 (4.00)
261 (4.43) 265 (4-45) 259 (4.57)
380 (1.89) 382 (1.86) 392 (1.74)
ET CH CY d
225 14-191
256 (4.30)
295 (4.44) 296 (4.43) 285
385s (2.04) 385s (2.05) 392
225
253
ET CH CY a
222(3-91)
258(3.97) 254
295 (4.11) 293 (4-47) 286
385s (1.78) 385s (2-04) 392
225
268 (4.52) 270 (4-48) 271 (4-48)
334s (2.77) 334s(2.56)
401s (1.96) 392 (2-03) 415 (4.48)
295 (4.21) 300 (4.20) 294
408 (1.74) 400s (1.79) 412
299 (4.02) 299 (4.(15) 296 (4.1121
41~)s
223 (4-26)
ET CH CY
V
ET CH CY 't
22414-01)
V!
ET CH CY
232 (3-67)
VII
ET CH CY
225
224(3.89)
409 (1.791
304 (4.03) 306 14-18) 302 (4.26)
405 (2.08) 427 (1.94)
VIII
ET CH CY
257 (4.32)
266 (4-34) 268(4-64)
324 (3.50) 325 (3.67) 321
418 (3.03) 420 (3-23) 400
IX
ET CH CY
257(4-64) 259 (4.56) 256(4.8111 40(1 (2.16)
274(4.11) 27914.(181 275 (4.23) 420 (2,00)
328 (3.63) 334 (3.55) 322 (3-72)
400 (2-23) 400 12.121 377 (2.32)
"Compounds are designcd according to Tablc 1. b ET: cthanol; CH: chloroform: CY: cyclohcxanc. ~ s: shoulder. ,t Partially dissolved; the saturated solution was measured.
i
~3.0 o
I~ 2.0
1.0
I
40
I
30 v x 103(cm -1)
I
20
Fig. 1. Absorption spectra of IV in (1) cyclohexane and (2) ethanol. 1-polar solvent complex. 15 VII exhibits a very distinct n - : r * band in cyclohexane. In ethanol it is completely lost. Similarly to Vll, the compound 1V loses its low intensity band in polar solvents (Fig. 1), but a small shoulder is preserved, probably due to the second chromophoric carbonyl group. The compounds IV and VII should form a weak complex with polar solvents even more easily than 1.15 Other absorption bands are more intense and less sensitive to solvent polarity. A slight bathochromic shift is observed in going from nonpolar to polar solvents. This behaviour is also observed for the 260 nm band. In contrast, Vlll, which is a cyclic 1,2diketone, exhibits a longest wavelength band of higher intensity (e-- 103 dm 3 mo1-1 cm -~) than acyclic 1,2-diketones. In going from nonpolar to polar solvent the bathochromic shift of this band is observed in solution as well as in the polymer matrix. In this respect its intensity is higher than for the typical n - : r * transition of acyclic 1,2-diketones. 1,2-Diketones exhibit similar absorption spectra in polymer matrices and solvents of similar polarity. No aggregation was observed in the absorption spectra in our concentration range of doping of 1,2-diketones in polymer matrices. Since the differences in polarity of polymer matrices are low, the differences in absorption spectra are low as well. Therefore, the differences in photoreactivity of 1,2-diketones in
198
Pavol Hrdlovi~, Ivan Lukd~
various matrices are not expected to be large either. Emission spectra Simple acyclic 1,2-diketones exhibit measurable emission in deaerated solution at laboratory temperature) At low temperature (77 K) in the solid solution, they yield very intense emission. For optical measurements of this type, PMMA constitutes a suitable environment. PMMA films of 50 # m thickness are mechanically stable and they withstand sudden temperature changes. On the other hand, control of impurities and oxygen seems to be a problem. Therefore, it is difficult to compare absolute intensities of compounds doped in a polymer film. It is appropriate to compare the intensity of emission of a series of compounds with the intensity of emission of a selected compound which is not sensitive to environment and quenching. Anthracene might be a suitable standard. At front face arrangement and high absorption at the excitation wavelength, the relative quantum yield, given as the ratio of integrals of emission bands of the compound under study and a standard, does not depend on the condition of measurement. Thus, the comparison of relative intensities of emission can yield important information. Emission spectra of 1,2-diketones of the benzil type at 77 K and at laboratory temperature are simple and exhibit a broad band (Fig. 2). Derivatives of 1-phenylpropane-l,2-dione exhibit another rather weak band around 570 nm (Table 3). In this region there is a maximum of the single emission band of cyclic VIII (Fig. 2). The excited triplet state of VIII has the same structure as the ground state, namely c/s planar, because it is fixed by the frame of the phenanthrene. On the other hand, the structure of the lowest triplet state of acyclic 1,2dicarbonyl derivatives is different, namely trans planar. ~6 Recently, Mukai et al. ~7 have classified three different configurations for the first triplet state and found a correlation between the parameters of zero field splitting and the maximum of phosphorescence. According to their interpretation the longest wavelength phosphorescence at 570 nm originates in a trans planar structure. This means that the emission from T~ of trans and c/s planar structure is similar at 570 nm due to extensive conjugation. Emission at lower values of ~,m,x stems from structures
II
tl
II I\i i / / I! i
500 ;~ (nrn)
400
600
Fig. 2. Emission spectra of (1) !I!, (2) VI and (3) VIII in PMMA at 77 K.
Table 3. E m i s s i o n spectra of 1,2-diketones in PMMA film at 77 K and laboratory temperature Compound ~
Temp. (K)
;t.,ax (nm)
dprc
MLT/r
a
(ms)
I
Ith 77
544 545,576
0.010 0-116
0-12 5-5
11
It 77
519
0.035 1.393
0.27 3.8
111
It 77
533 522
0.037 1-984
0-18 4.1
IV
It 77
544 529
0-032 2.201
0.42 5.6
V
it 77
527,527 524, 546
0-064 0.659
0-48 2.3
V!
It 77
529 528,561
0-063 0.567
0.49 2-3
VII
It 77
517, 566 519,566
0.039 0.879
0.19 5.4
ViII
It 77
394,403,546,574 569
0-038 0"802
9.0
iX
It 77
428, 438,469, 492,529 445,490, 526
0-078 2-483
X
it 77
418, 438,449,453,470 414,448,476, 510
0-01 2.017
= Designation of compounds according to Table 1. b It: laboratory temperature (293-298 K). c Quantum yield relative to the emission of anthracene in PMMA film (50 #m) at 0.1 mol kg -I concentration at 313 nm excitation. a MLT: mean lifetime of emission at laboratory temperature which was fitted to a biexponential; the decay of emission at 77 K was fitted to a monoexponential.
199
Derivatives of 1, 2-diketones as initiators of polymer degradation Table 4. Emission spectra of 1,2-Diketones in PE a Compound
T e m p . (K)
!
It 77
531
0.012 0.855
5.2
It 77
545 524
0-016 5.15
3.4
It 77
512
0-027 3.695
4.2
it 77
550
0.009 2.010
4-3
It 77
528,566
0.027 1.480
1.4
It 77
530,575
0-008 0.690
1.8
it 77
522,572
0.003 0.565
3.5
I! !!I IV V V! 400
500 ~. (nm)
600
VII
;tm~x(nm)
Cr h
zc (ms)
Fig. 3. Emission spectra of V I I in (1) P M M A at laboratory temperature, (2) P M M A at 77 K and (3) P E at 77 K.
See footnotes to Table 3. b Q u a n t u m efficiency relative to emission of anthracene. c Decay of emission at 77 K was fitted to a monoexponential.
where the two benzoyl groups are mutually rotated. Consequently, we cannot identify any T~ state of the 1,2-diketones under study, in any matrix, as trans planar, because all Am~x were lower than 570 nm. The compound IV exhibited the highest ~max. This implies that all structures of T~ have got some value of angle between the two benzoyl groups, which lies between 0° and 90 °. This angle changes more or less continuously because it was not possible to identify clearly any distinct states. It is of some interest to note that compounds II! and VII exhibit low ~'max in PE and PMMA matrices, indicating a rather large angle between benzoyl groups, which might be due to some specific interactions. In going from laboratory temperature to 77 K, a small hypsochromic shift and a narrowing of the emission bands is observed (Fig. 3). This again implies that the emission at 77 K originates from structures with a larger angle between the planes of the benzoyl groups. The quantum yields of 1,2-diketone emission relative to anthracene are rather low at laboratory temperature (Tables 3 and 4). On the other hand, the emission at 77 K is comparable with that of anthracene and in some cases even higher, especially in PE matrix. At the moment it is difficult to decide whether the increase in emission intensity is due to suppressing some specific radiationless process or lack of quenching by impurities or oxygen at 77 K.
Values of ~, for 1,2-diketones in the PE matrix at laboratory temperature are very low (Table 4). This indicates that oxygen quenching is the dominating process, leading to the decrease in intensity. This conclusion supports the fact that the intensities from 1,2-diketones in PMMA and PE are similar at 77 K. At laboratory temperature the difference in intensities of 1,2-diketones is caused by different extents of oxygen quenching in polycrystaUine PE and glassy PMMA. In Table 5, the maxima of emission in various matrices are given. The general trend in these values is clearly seen for derivatives of IV and V. In going from nonpolar to polar matrices a Table 5. The influence of polymer matrix on maximum of emission of 1,2-diketones at 77 K ° Compound
I n nl iv v v! vii viii
Matrix h PE
iPP
PS
531 524 512 550 528 530 522
543 527 533 540 528 528 522
539 526 535 538 530 528 522 577
"See footnotes to Table 3. b See experimental section.
PMMA (~,. . . . nm) 545 519 522 529 524 526 519 567
PVC
545 518 518 534 525 525 518 572
PVA
519
523
200
Pavol Hrdlovi6, loan Lukd~
hypsochromic shift of the emission maxima is observed. For derivative IV this shift is about 20 nm. For other derivatives it is 5 to 10 nm only. There are, however, some deviations from this general trend, especially for III and I. The values of '~'max for these compounds are rather low as one would expect for a PE matrix. This means that these compounds adopt a structure where the angle between benzoyl groups is for some reason larger. In the polar matrix PVA, only the emission of II and V can be measured because the other derivatives are not soluble in water-ethanol mixtures. In this polyhydroxylic matrix the emission was similar to the ones in other polymer matrices except for a small hypsochromic shift. No emission below 450 nm for these compounds was observed as reported by Inoue et al. 18 for so-called 'bridged benzil' (10,11-dihydro-5H-dibenzo-[la, d]-cycloheptane10,11-dione) in a solid ethylene glycol matrix. We assume that the hypsochromic shift of emission in going from nonpolar to polar matrix is a polarity effect. The anomalies in the position of the emission maximum are due to matrix rigidity or some other specitic interactions. The decay of the emission at 77 K in PE and PMMA is monoexponential, covering two orders of magnitude in intensity. The correlation coefficient is usually better than 0-99 and the 'goodness of fit', expressed as the standard deviation of experimental and calculated curves is better than 5%. In the PE matrix the lifetimes are slightly shorter than in PMMA, which might indicate some contribution of static quenching. The lifetimes of 1-phenylpropane-l,2-diones are shorter, in the range 2-4 ms, as compared with I derivatives, which are in the range 3-6 ms. Since the measurable emission is observed in the P M M A matrix at laboratory temperature only, the decay of emission was analysed in this case only. The decay of emission was not monoexponential. It was well fitted by a biexponential decay. The 'goodness of fit', expressed as the standard deviation of experimental and calculated curves, is better than 2%. The mean lifetime lies in the range 0.1-0-5 ms, which is one-tenth of the value at 77 K. In this case, derivatives of 1phenylpropane-l,2-dione exhibit longer mean lifetimes than benzil-type derivatives. The decay was also fitted to the so-called 'stretched exponential'. The fit was, however, worse than for the biexponential. It is not quite clear which
temperature dependent radiationless process causes the decay to become different at different sites in the polymer matrix. It is possible that the accesibility of different sites, even in amorphous PMMA, to quencher (oxygen) varies. This has been recently observed for the quenching of phenanthrene in P M M A matrix by oxygen. 19 There is some distribution of the rate of quenching, but the biexponential could be a good approximation. Consequently, the decay of emission in polymer matrices at laboratory temperature is a complex phenomenon.
Photooxidation of polymers Derivatives of 1,2-diketones of both types are initiators of polymer photooxidation. They act as promotors of oxidation, mainly at the beginning (Fig. 4). In later stages of oxidation the process becomes slower. This course of oxidation is similar to that of other types of photoinitiator, e.g. derivatives of 9,10-anthraquinone.~° In order to compare various photoinitiators in different polymer substrates, it is convenient to define the effectiveness of acceleration. This is the ratio of the times required to reach a given absorption in a selected spectral region in polymer without and with initiator. A value larger than 1 means acceleration. If oxidation slows down at later stages, then the relative acceleration is influenced by the chosen limit. If AAco is chosen in the range 0.05-0.2, then the conclusions about the effectiveness of a series of photoinitiators might be quite reasonable. Comparison of relative values of acceleration in Table 6 shows that the derivatives of 6 0.50.4-
3 •
o
2 • 0.3 0.2 0.I
01
I00
i 200 t(h)
I 300
Fig. 4. The course of photooxidation of PE at laboratory temperature (1) without photoinitiators, and with (2) II!, (3) Vi, (4) IV, (5) VIII and (6) IX.
Derivatives of 1, 2-diketones as initiators of polymer degradation Table 6. Relative efficiency of 1,2-diketones and other initiators on photooxidation of polymers"
Compound
Matrixh PE
Without i ii iil
IV V ~i iX
X
iPP PS (relative efficiency)
201
less effective in P V C only. Similar effectiveness is exhibited by VII. On the other h a n d , derivatives with free hydroxy groups exhibit low effectiveness.
PVC
1.0 1-6
1.0 1-2
1.0 2.1
1.0 0.9
2-6
0-8
1.3
2.5
1-5 4.4 1.6 1.6 4-4 6.8 3.2
0-8 1.7 0.8 0.8 1.4 1.9 1.9
1.3 3.5 4-2 1.4 3.2 16-4 5-0
1.7 1.8 2.5 0.6 3.6 3.4 3.6
"See footnotes to Table 3. h PE exhibited absorption of carbonyl band AA~o= 0-2 after 300h, iPP after 70h, PVC after 375 h and PS exhibited AA~, =0.1 after 575 h.
1,2-diketones are less effective than 2-methylanthraquinone. For inherently more stable polymers like PS, the relative effectiveness is higher than in the more sensitive iPP. In this substrate, the effectiveness is even lower than 1 for four substrates. This means that, instead of acceleration, weak stabilization takes place. On the other h a n d , the stabilization effect is low, although the absorption in the n e a r - U V region is rather high. This is due to the fact that 1,2-diketone decomposes at the beginning rapidly. This is evident from I R spectra. The further course of degradation depends on the products formed, which are given by the type of 1,2-diketone and type of polymer. The most effective acyclic 1,2-diketone is the bichromophoric c o m p o u n d IV, which is slightly
REFERENCES
1. McGinniss, V. D., In Developments in Polymer Photochemistry, ed. N. S. Allen. Vol. 3, 1982, p. 1. 2. Fouassier, J. P., In Photochemistry and Photophysics, ed. J. F. Rabek. CRC Press, Boca Raton, FL, 1990, Vol. 2, p. 1. 3. Backstrom, H. J. L. & Sandros, K., Acta Chem. Scand., 14 (1960) 46. 4. Dan6i~ek, J., Hrdlovi~, P. & Luke,, I., Eur. Polym. J., 12 (1976) 513. 5. Rubin, M. B., Top. Curr. Chem., 129 (1985) 1. 6. Horie, K. & Mita, I., Macromolecules, 11 (1978) 1175. 7. Luk~i~,I., Zvara, I. & Hrdlovi~, P., Eur. Polym. J., 18 (1982) 427. 8. Gebert, M. S. & Torkelson, J. M., Polymer, 31 (1990) 2402. 9. Hrdlovi(~,P., Taimr, L. & Pospi~il, J., Polym. Deg. and Stab., 25 (1989) 73. 10. Luke,., I. & DasMohapatra, G. K., Collect. Czech. Chem. Commun., 57 (1992) 1082. 11. Vogel, A. I., Practical Oraganic Chemistry, Longman, London, 1956, p. 835. 12. Moyze, G., Ml~nek, J., Jur6~ik, D. & Hrdlovi~, P., Chem. Listy, 86 (1992) 57. 13. Hrdlovi~, P., Polym. Photochem., 7 (1986) 359. 14. Hrn~irik, F., Chem. PrCtm., 31/56 (1981) 594. 15. Miata, K. & Koyanagi, M., Bull. Chem. Soc. Jap., 61 (1988) 3813. 16. Morantz, D. J. & Wright, A. C. J., J. Chem. Phys., 54 (1971) 523. 17. Mukai, M., Yamauchi, S. & Hirota, N., J. Phys. Chem., 96 (1992) 9328. 18. lnoue, H., Sakurai, T., Hoshi, T., Okubo, J. & Kawashima, T., J. Chem. Soc., Faraday Trans., 2 (1986) 523. 19. Bagryansky, V. A., Korelev, V. V., Tolkatchev, V. A. & Bazhin, N. M., J. Polym. Sci., B: Polym. Phys., 30 (1992) 951.